76 
Butler 
of the corpus callosum to determine whether 
or not this possibility is feasible anatomic- 
ally. 
As was discussed above, Elliot Smith 
(1910) concluded that the dorsal commissure 
in reptiles, monotremes, and marsupials is 
the hippocampal commissure. Abbie (1939) 
working with brains from adult mammals, 
also presented a strong argument that the 
corpus callosum is a de novo structure in 
eutherian mammals, one of his points being 
that the corpus callosum does not develop in 
the lamina terminalis, but is related to it only 
secondarily. He believed that in development 
the callosal fibers “break through the 
subiculum” and that “the upper edge of the 
lamina terminalis [serves as] the necessary 
bridge, for the passage of fibers between the 
hemispheres.” Streeter (1907), however, in 
his studies of human embryos, concluded that 
the corpus callosum does in fact develop 
within the lamina terminalis. 
The mechanism of callosal development, 
partly obscured in the literature due to 
terminological differences, was finally clari- 
fied by Rakic and Yakovlev (1968). From 
study of human embryological material, they 
demonstrated that the corpus callosum and 
hippocampal commissure both develop in the 
same part of the lamina reunions of His, a 
dorsal structure included descriptively by 
Streeter (1907) and others as part of the 
lamina terminalis. 
Thus, were any cell groups in the reptilian 
dorsal cortex shown to be homologous to cell 
groups in the mammalian neocortex, any 
interhemispheric connections of these groups 
traveling in the dorsal pallial (hippocampal) 
commissure could correspond to the mam- 
malian corpus callosum. Of course no conclu- 
sions can be drawn regarding interhemi- 
spheric connections in reptiles until the 
nature of the various cortices, and dorsal 
cortex in particular, is understood. 
In addition to the questions regarding the 
dorsal cortex, there is a great deal of infor- 
mation on the organization of the dorsal 
ventricular ridge yet to be gained. In par- 
ticular, detailed work is necessary on the 
organization of afferent and intrinsic projec- 
tions in relation to the cytoarchitectonic 
arrangement of the DVR in type I and type 
II lizards, as well as in other reptiles and 
birds which have either the corticoid band- 
core arrangement or the nuclear arrange- 
ment of cells in the DVR. Such an analysis 
can provide insights into principals of neural 
organization in the vertebrate nervous sys- 
tem, by recognition of which characters are 
generalized, and into the relationship of 
morphology to evolution and function, by 
identification of which characters are spe- 
cializations for particular adaptive zones. 
ACKNOWLEDGMENT 
This work was supported by the National Insti- 
tutes of Health Grants Nos. 1 ROl NS-11686-01 and 
9 ROl NS-12966-01. 
REFERENCES 
Abbie, A. A. 1939. The origin of the corpus callosum 
and the fate of the structures related to it. J. 
Comp. Neurol. 70:9-44. 
Ariens Kappers, C.U., G.C. Huber, and E.C. Crosby. 
1936. The Comparative Anatomy of the Nervous 
System of Vertebrates, Including Man, Hafner 
Publishing Company, New York. 
Armstrong, J.A. 1950. An experimental study of 
the visual pathways in a reptile {Lacerta 
vivipara). J. Anat., 84:146-167. 
Braford, M.R., Jr. 1972a. Ascending efferent tectal 
projections in the South American spectacled 
caiman. Anat. Rec. 172:275-276. 
Braford, M.R., Jr. 19726. Ipsilateral retinal pro- 
jections in non-mammalian vertebrates. Am. Zool. 
12:728. 
Braford, M.R., Jr. 1973a. Degeneration patterns 
following lesions of the olfactory bulb in Poly- 
pterus. Anat. Rec. 175:276-277. 
Braford, M.R. Jr. 19736. Retinal projections in 
Caiman crocodilus. Am. Zool. 13:1346. 
Braford, M.R., Jr., and R. Glenn Northcutt. 1974. 
Olfactory bulb projections in the bichir, Poly- 
pterus. J. Comp. Neurol. 156:165-178. 
Butler, A.B. 1976. The telecephalon of the lizard 
Gekko gecko (Linnaeus) : Some connections of the 
cortex and dorsal ventricular ridge. Brain, Behav. 
Evol. 13:396-417. 
Butler, A.B., and F.F. Ebner. 1972. Thalamo- 
telencephalic projections in the lizard Iguana 
iguana. Anat. Rec. 172:282. 
Butler, A.B., and R. Glenn Northcutt. 1971a. Ret- 
inal projections in Iguana iguana and Anolis 
carolinensis. Brain Res. 26:1-13. 
Butler, A.B., and R. Glenn Northcut. 19716. As- 
cending tectal efferent projections in the lizard 
Iguana iguMna. Brain Res. 36:697-601. 
